Method and apparatus for airway compensation control

ABSTRACT

A ventilator for ventilating a patient also assists a clinician in determining a suitable PEEP for the patient. For this purpose, a graph or tabular display of a series of different value PEEPs and corresponding functional residual capacities of the patient may be provided. Or, the relationship between lung compliance and a series of different values of PEEP may be provided. Or, the amount of the lung volume recruited/de-recruited at various levels of PEEP may be determined for use in selecting a desired PEEP. To this end, the functional residual capacity of the lungs is determined for a first PEEP level. The PEEP is then altered to a second level and a spirometry dynostatic curve of lung volume and pressure data is obtained. The lung volume on the dynostatic curve at a lung pressure corresponding to the first PEEP value is obtained. The difference between the functional residual capacity of the lungs at the first PEEP level and that determined from the dynostatic curve represents the lung volume recruited/de-recruited when changing between said first and second PEEPs.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the priority of U.S. ProvisionalApplication No. 60/719,329, filed Sep. 21, 2005, and comprises acontinuation-in-part of U.S. patent application Ser. No. 11/358,573,filed Feb. 21, 2006, which application also claims priority of U.S.Provisional Application No. 60/719,329.

BACKGROUND AND SUMMARY

The present invention relates to an apparatus and method for determiningand displaying functional residual capacity data and other pulmonaryparameters, such as positive end expiratory pressure (PEEP) data, forpatients breathing with the aid of a mechanical ventilator, such as acritical care ventilator. The invention also determines and displaysrelationships between these and other parameters.

Functional residual capacity (FRC) is the gas volume remaining in thelungs after unforced expiration or exhalation. Several methods arecurrently used to measure functional residual capacity. In the bodyplethysmography technique, the patient is placed in a gas tight bodybox. The patient's airway is sealingly connected to a breathing conduitconnected to the exterior of the body box. By measuring lung pressuresand pressures in the box, at various respiratory states and breathinggas valve flow control conditions, the functional residual capacity ofthe patient can be determined.

Another technique for measuring functional residual capacity is thehelium dilution technique. This is a closed circuit method in which thepatient inhales from a source of helium of known concentration andvolume. When the concentration of helium in the source and in the lungshas reached equilibrium, the resulting helium concentration can be usedto determine the functional residual capacity of the patient's lungs.

A further technique for determining functional residual capacity is theinert gas wash-out technique. This technique is based on a determinationof the amount of gas exhaled from the patient's lungs and correspondingchanges in gas concentrations in the exhaled gas. The gas used for themeasurement is inert in the sense that it is not consumed by metabolicactivity during respiration. While a number of gases may be used forsuch a measurement of functional residual capacity, it is convenient touse nitrogen for this purpose.

In a straightforward example in which the patient is initially breathingair, the lung volume forming the functional residual capacity of thelung will contain nitrogen in the same percentage as air, i.e.approximately 80%, the remaining 20% of air being oxygen. In a wash-outmeasurement, the subject commences breathing gases in which oxygen is ata different concentration than 20%. For example, the patient commencesbreathing pure oxygen. With each breath, nitrogen in the lungs isreplaced by oxygen, or, stated conversely, the nitrogen is “washed out”of the lungs by the oxygen. While the breathing of pure oxygen couldcontinue until all nitrogen is washed out of the lungs, in most cases,the breathing of oxygen continues until the nitrogen concentration inthe exhaled breathing gases falls below a given concentration. Bydetermining the volume of inert gas washed out of the lungs, and knowingthe initial concentration of the inert gas in the lungs, the functionalresidual capacity of the lungs may be determined from these quantities.

Methods for determining functional residual capacity in this manner arewell known and are described in such literature as The BiomedicalEngineering Handbook, CRC Press, 1995, ISBN 0-8493-8346-3, pp.1236-1239, Critical Care Medicine, Vol. 18, No. 1, 1990, pp. 8491, andthe Yearbook of Intensive Care and Emergency Medicine, Springler, 1998,ISBN 3-540-63798-2, pp. 353-360. By analogy to the above described washout measurement technique, it is also possible to use a wash in of inertgas for measurement of functional residual capacity. Such a method andapparatus is described in European Patent Publication EP 791,327.

The foregoing methods are used with spontaneously breathing patients andare typically carried out in a respiratory mechanics laboratory. But inmany cases, patients that could benefit from a determination offunctional residual capacity are so seriously ill as to not be breathingspontaneously but by means of a mechanical ventilator, such as acritical care ventilator. This circumstance has heretofore proven to bea significant impediment in obtaining functional residual capacityinformation from such patients. Additionally, the patient's illness mayalso make it impossible or inadvisable to move the patient to alaboratory or into and out of a body box for the determination offunctional residual capacity.

It would therefore be highly advantageous to have an apparatus andmethod by which the functional residual capacity of mechanicallyventilated patients could be determined. It would be furtheradvantageous to associate the apparatus for carrying out thedetermination of functional residual capacity with the ventilator toreduce the amount of equipment surrounding the patient and to facilitateset up and operation of the equipment by an attending clinician. Suchapparatus would also enable the determination of functional residualcapacity to be carried out at the bedside of the patient, thus avoidingthe need to move the patient.

A single determination of functional residual capacity providesimportant information regarding the pulmonary state of the patient.However, it is often highly desirable from a diagnostic or therapeuticstandpoint to have available trends or changes in the functionalresidual capacity of a patient over time.

It would also be helpful to be able to relate functional residualcapacity to other pulmonary conditions existing in the lungs orestablished by the ventilator and to changes in these conditions. Forexample, it is known that the pressure established by the ventilator inthe lungs at the end of expiration, the positive end expiratory pressureor PEEP, affects the functional residual capacity of the lungs.

Typically, an increase in PEEP increases functional residual capacity.There are two components to the increased functional residual capacityas PEEP is increased. One component is due to stretching of the lung bythe increased pressure. A second component, particularly in diseasedlungs, occurs from the effect of PEEP during breathing by the patient.As a patient expires, the pressure in the lungs drops until itapproaches airway pressure. As the pressure within the lungs drops, thealveoli or air sacs in the lungs deflate. If alveolar sacs collapsecompletely, more pressure is required upon inspiration to overcome thealveolar resistance and re-inflate the alveolar sacs. If this resistancecannot be overcome, the volume of such sacs are not included in thefunctional residual capacity of the patient's lungs.

By applying PEEP, in the patient's airway, the additional pressure inthe patient's lungs keeps more of these alveolar sacs from completelycollapsing upon expiration and, as such, allows them to participate inventilation. This increases the functional residual capacity of thepatient's lungs and the increase is often described as “recruitedvolume.” Volume reductions are termed “de-recruitments.”

However, setting the PEEP too high can cause excessive lung distension.There may also be compression of the pulmonary bed of the lung, loadingthe right side of the heart and reducing the blood volume available forgas exchange. Either of these circumstances present the possibility ofadverse consequences to the patient.

It would, therefore, be desirable to provide an apparatus and method bywhich a clinician could quickly, easily, and definitely determine anoptimal PEEP for a given patient at a given point in the therapeuticregimen for the patient. An optimal PEEP is one that keeps the lung openbut avoids overpressurization of the lung. It is often termed the “openlung PEEP.”

Still further, action such as performing a suction routine,administering a nebulized medication, or changing the ventilationparameters of the ventilator can also influence functional residualcapacity and it would be helpful to be able to easily determine theeffect of such actions on functional residual capacity.

An apparatus and method that would possess the foregoing characteristicsand that would easily and cogently make such information available wouldbe highly beneficial in conveniently obtaining a full understanding ofthe pulmonary condition of the patient and how the patient is reactingto the mechanical ventilation and to any associated therapeuticmeasures. The clinician could then carry out appropriate actionbeneficial to the patient in a timely and informed manner.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

An embodiment of the present invention comprises an apparatus and methodthat achieves the highly advantageous features noted above. Thus, withthe present invention the functional residual capacity of a mechanicallyventilated patient may be determined at the bedside of the patientwithout the need to move the patient to a laboratory. By associating theapparatus with the ventilator, only a single device need be employed toboth ventilate the patient and determine functional residual capacity.

The determined functional residual capacity may be advantageouslydisplayed in conjunction with earlier determinations and in conjunctionwith other pulmonary conditions, such as PEEP. Changes, or trends, infunctional residual capacity over time may thus be discerned, along withchanges in the other pulmonary conditions.

The foregoing provides an attending clinician with significantinformation for assessing the state of, and trends in, the functionalresidual capacity of the patient, as well as the relationship betweenthe patient's residual capacity and the other factors, so that theclinician can fully discern the functional residual capacity conditionof the patient.

With respect to assisting the clinician in adequately determining anoptimal PEEP for the patient, as noted above, the apparatus and methodof the present invention determines and displays related PEEP andfunctional residual capacity values. This enables the clinician to note,for example, the point at which increases in PEEP produce little, ifany, further increases in functional residual capacity.

The apparatus and method of the present invention also determines anddisplays a showing of the amount of lung volume recruited orde-recruited as the PEEP is changed. This allows the clinician todistinguish between changes in functional residual capacity due to lungstretching or contracting and those arising from recruitment orde-recruitment.

Further features of the apparatus and method of the present inventionwill be apparent from the following detailed description, taken inconjunction with the associated drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general diagram of a mechanical ventilator and associatedapparatus for ventilating a patient.

FIG. 2 shows an endotracheal tube with a tracheal pressure sensorsuitable for use in the present invention.

FIG. 3 shows a ventilator display unit presenting an initial displayscreen for use in the present invention.

FIG. 4 is a chart showing the relationship among a plurality of screensemployed in the present invention.

FIG. 5 shows a display screen for displaying functional residualcapacity data and related data.

FIG. 6 shows a display for use in scaling the display shown in FIG. 5.

FIG. 7 is a flow chart showing the steps for carrying out a measurementof functional residual capacity.

FIG. 8 shows a display displaying a log of events and actions that mayimpact the determination of functional residual capacity.

FIG. 9 shows a display showing the effect of changes in PEEP onfunctional residual capacity.

FIG. 9A shows a display showing changes in PEEP and functional residualcapacity before and after a recruitment maneuver.

FIG. 9B shows a display showing the relationship of changes in PEEP tocorresponding changes in functional residual capacity.

FIG. 10 shows a display showing spirometry data.

FIG. 11 shows a display for making setup adjustments for the screenshown in FIG. 10.

FIGS. 12 a and 12 b show a display showing relationships amongfunctional residual capacity, PEEP, and recruited lung volume.

FIG. 13 is a flow chart showing the steps for carrying out a method forobtaining the relationships shown in FIGS. 12 a and 12 b.

FIG. 14 is a graph showing the manner in which recruited/de-recruitedvolume is determined by the present invention.

DETAILED DESCRIPTION The Mechanical Ventilator and Airway Gas Module

FIG. 1 shows mechanical ventilator 10 for providing breathing gases topatient 12. Ventilator 10 receives air in conduit 14 from an appropriatesource, not shown, such as a cylinder of pressurized air or a hospitalair supply manifold. Ventilator 10 also receives pressurized oxygen inconduit 16 also from an appropriate source, not shown, such as acylinder or manifold. The flow of air in ventilator 10 is measured byflow sensor 18 and controlled by valve 20. The flow of oxygen ismeasured by flow sensor 22 and controlled by valve 24. The operation ofvalves 20 and 24 is established by a control device such as centralprocessing unit 26 in the ventilator.

The air and oxygen are mixed in conduit 28 of ventilator 10 and providedto inspiratory limb 30 of breathing circuit 32. Inspiratory limb 30 isconnected to one arm of Y-connector 34. Another arm of Y-connector 34 isconnected to patient limb 36. During inspiration, patient limb 36provides breathing gases to lungs 38 of patient 12. Patient limb 36receives breathing gases from the lungs of the patient duringexpiration. Patient limb 36 may include components such as a humidifierfor the breathing gases, a heater for the breathing gases, a nebulizer,or a water trap (not shown). The breathing gases expired by patient 12are provided through patient limb 36 and Y-connector 34 to expiratorylimb 46 of breathing circuit 32. The expired breathing gases inexpiratory limb 46 are provided through valve 54 and flow sensor 56 fordischarge from ventilator 10. Valve 54 may be used to establish the PEEPfor patient 12.

Patient limb 36 includes gas flow and pressure sensor 57 which may be ofthe type shown in U.S. Pat. No. 5,088,332. A pair of pressure ports andlines 58, 60 are placed on either side of a flow restriction in thesensor and the pressure difference developed across the flow restrictionis used by flow measurement unit 62 in gas module 64 to measure gas flowin patient limb 36. One of the pressure lines is connected to pressuremeasurement unit 66 to measure the pressure in patient limb 36. Sensor57 also provides for a gas sampling line 68 which is connected to gasanalyzer 70. Gas analyzer 70 may measure the amount of oxygen and carbondioxide in the breathing gases. Knowing the amounts of oxygen and carbondioxide in the breathing gases enables the amount of nitrogen to bedetermined as the total amount less the amounts of carbon dioxide andoxygen. Respiratory and metabolic gas module 64 may comprise that madeand sold by GE Healthcare as a Datex-Ohmeda MCOVX gas module. The outputof gas module 64 is provided in data bus 72 to central processing unit74 in ventilator display unit 76. Central processing unit 26 inventilator 10 is also connected to central processing unit 74 via databus 78.

The Endotracheal Tube

To obtain an accurate indication of the pressure in lungs 38 of thepatient 12, endotracheal tube 90 shown in FIG. 2 may be used.Endotracheal tube 90 has end 92 for connection to patient limb 36. Inuse, endotracheal tube 90 extends through the mouth and into the tracheaof patient 12 to provide an airway passage to lungs 38.

Endotracheal tube 90 includes pressure sensor catheter 94 that extendsfrom end 96 to provide a pressure sampling point that is close to lungs38 of patient 12 when the endotracheal tube is inserted in the patientand can thus obtain a highly accurate indication of the pressure in thelungs. An intermediate portion of catheter 94 may lie withinendotracheal tube 90. The proximal portion exits the endotracheal tubeand is connected via A-A to a pressure transducer and to an auxiliaryinput to ventilator display unit 76. The pressure obtained from catheter94 is termed Paux. While FIGS. 1 and 2 show a connection to ventilatordisplay unit 76 for this purpose, the connection may, alternatively, beto gas module 64.

An endotracheal tube of the type shown in FIG. 2 is described in U.S.Pat. No. 6,315,739.

Ventilator Display Unit

Display unit 76 of ventilator 10 receives information from theventilator and gas module 64 and is used by the clinician to control thepneumatic control components of ventilator 10 that deliver breathinggases to patient 12 via data bus 78. Additionally, central processingunit 74 in display unit 76 carries out the determination of functionalresidual capacity, recruited/de-recruited volumes, and other quantitiesemployed in the present invention. It will be appreciated that other CPUconfigurations, such as a single CPU for the ventilator and its displayunit may be used, if desired.

Ventilator display unit 76 includes user interface 100 and display 102.Display 102 is shown in greater detail in FIG. 3. Display 102 is dividedinto a number of display portions 102 a-g for displaying inputted,sensed, and computed information. Display portions 62 a though 102 frelate primarily to the operation of ventilator 10 and the ventilationof patient 12 and are discussed briefly below. Display screen portion102 g displays information and relationships in accordance with thepresent invention, as described in detail below.

Display portion 102 a provides for the display of operating informationof ventilator 10. The portion shows the type of ventilation beingperformed by ventilator 10, in the exemplary case of FIG. 3,synchronized, intermittent, mandatory ventilation, or SIMV-volumecontrolled ventilation. Portion 102 a also provides a display ofoperating information inputted into ventilator 10 including thepercentage of oxygen for the breathing gases, tidal volume (TV),breathing rate, inspiration time (T_(insp)), amount of positive endexpiratory pressure (PEEP) and the pressure limit (P_(limit)) set forthe volume controlled ventilation. To input these operating parametersinto ventilator 10, an appropriate one of buttons 104 a through 104 f isactuated. Control knob 106 is rotated to enter a desired value for theselected option and pressed to confirm the new parameter value. Furtherventilator functions may be controlled by pressing a button thatcontrols a specialized function such as ventilator setup button 72 thatestablishes other ventilation modes for patient 12, spirometry button 74for showing and controlling the display of spirometry information, 100%O₂ button 76, nebulizer button 78, and procedures button 80 thatcontrols specialized procedures for ventilator 10.

Display portion 102 b of display 102 shows airway pressure data asmeasured from sensor 57. Portion 102 c shows textual informationrelating to the flow of breathing gases to the patient obtained fromsensor 57, and portion 102 d shows pressure data from catheter 94 in theendotracheal tube 90 during ventilation of patient 12.

Portion 102 e of display 102 shows the information in regions 102 b, 102c, and 102 d in graphic form and includes an indication of certain otheroperating information, such as the mode of ventilation SIMV-VC, andwhether certain features of the present invention are operational ornot.

Display portion 102 f of display 102 shows additional data as selectedby the clinician. In the example of FIG. 3 end tidal CO₂ (E_(t)CO₂),lung compliance, expiratory alveolar minute volume (MVe (alv)),respiratory rate, total positive end expiratory pressure, andinspiratory alveolar minute volume (MVi (alv)) are being shown.

Display portion 102 a-f remain generally unchanged as the presentinvention is practiced although, as noted above, the clinician mayselect the information to be shown in certain portions, such as portion102 f.

Display Screen of Present Invention

Display screen 102 g is the part of display 102 employed in the presentinvention. As shown in FIG. 4 and in FIGS. 5, 6, 8 and 9-12, the contentof this screen will change, depending on the inventive feature beingutilized, the different content in screen 102 g being identified as 102g 1, 102 g 2, 102 g 3, etc. in the appropriate figures of the drawing.

In general, each screen 102 g will include a menu or control portion108, a graphic portion 110 and tabular portion 112. For this purpose,graphic portion 110 contains a pair of orthogonal axes by which data canbe graphically presented. The clinician may navigate and control thescreen using control knob 106. Control knob 106 is rotated to scrollthrough the menu options displayed in menu portion 108, depressed toselect a menu option, rotated again to establish a numerical value forthe selected option when appropriate, and depressed again to enter thevalue into ventilator display unit 76 or to confirm selection of themenu option.

FIG. 3 shows an initial content for screen 102 g relating to spirometry.As hereinafter noted, spirometry illustrates the relationship betweeninspired gas volumes and the pressure in the lungs as the patientbreathes. The graphic form of the data is normally in a loop, a portionof which is formed during inspiration and the other portion of which isformed during expiration in the manner shown in FIG. 10. The tabularportion 112 provides fields in which various obtained and computedventilation and lung properties may be displayed.

Menu portion 108 allows the clinician to select a number of options withrespect to the display and use of the information shown in graphic andtabular portions 110 and 112. Menu portion 108 also allows the clinicianto select a further screen at 116 for adjusting the scaling for theabscissa and ordinate of graph 110 and the setup for spirometrymeasurements at 118.

From menu portion 108, the clinician may also select screens that allowthe functional residual capacity (FRC) features of the present inventionand the spirometry features of the present invention to be carried outby selecting items 120 and 122, respectively. The spirometry features ofthe present invention are identified by applicant as SpiroDynamics orthe abbreviation SpiroD.

FIG. 4 shows the architecture of the screens 102 g used in the presentinvention. As noted above, the spirometry screen shown in FIG. 3 asscreen 102 g 1 is the initial screen appearing as screen 102 g. As notedabove, associated with this screen are screens for spirometry scalingand spirometry setup.

By means of menu items 120 and 122, the clinician can select either ascreen relating to functional residual capacity, namely screen 102 g 2shown FIG. 5 or a screen relating to SpiroDynamics comprising screen 102g 3 of FIG. 10. The screen format of FIG. 5 is termed “FRC INview.” Theview of FIG. 10 is termed “spiroD”.

The FRC INview showing of 102 g 2 includes screen shown in FIG. 6 thatallows for scaling of the quantities shown graphically in FIG. 5.

A further selection on the FRC INview screen allows the clinician toselect the FRC log screen shown in FIG. 8 as screen 102 g 4.

Selections on either of the FRC INview screen 102 g 2 or theSpiroDynamic screen 102 g 3 allows selection of a PEEP INview screenshown in FIG. 9 as 102 g 5. As hereinafter described, this screen allowsthe clinician to see the relationship between functional residualcapacity and PEEP to assist in selecting an appropriate PEEP for patient12.

Finally, an on/off selection option in PEEP INview screen 102 g 5 allowsthe clinician to display lung INview screen 102 g 6 shown in FIG. 12.The information contained in this screen relates functional residualcapacity, PEEP, and recruited/de-recruited lung volumes to furtherassist the clinician in setting the appropriate level of PEEP.

FRC Determination and Display

The flow chart of FIG. 7 shows a method for determining and displayingfunctional residual capacity information for patient 12. The clinicianuses a screen in the format of 102 g 2 of FIG. 5. It is assumed that theclinician has previously established an oxygen percentage for thebreathing gases to be provided by ventilator 10 using button 104 a,control knob 106 and screen region 102 a, at step 200. In the exampleshown in FIG. 3, the oxygen percentage is 50%. Ventilator 10 can beoperated with the set percentage of oxygen to provide breathing gases topatient 12 at step 202.

As noted above, in order to determine the functional residual capacityof patient 12 by a gas wash-out/wash-in technique, it is necessary toalter the composition of the breathing gases supplied to patient 12. Tothis end, the clinician sets a different level for the oxygen content ofthe breathing gases. This is performed by selecting the FRC O₂ field 206in menu portion 68 of screen 102 g 1 and appropriately establishing theFRC O₂ value. The amount of change may be an increase or decrease fromthe previously set level established at step 200; however it must be anamount sufficient to perform the functional residual capacity analysis.A change of at least 10% is preferable in order to obtain an accurateindication of the functional residual capacity. To ensure thatappropriate oxygen concentrations are supplied to patient 12 it isusually desired to increase the oxygen level and, unless the currentoxygen level is very high (greater than 90%), a default setting of a 10%increase over the current setting may be provided. The level of oxygenset by the clinician “tracks” changes made in the oxygen content of thebreathing gases at the ventilator, as for example by actuating button104 a. Thus, for example, if the ventilator oxygen is originally 50% asshown in FIG. 3, and the FRC O₂ shown in FIG. 5 is 60%, if theventilator oxygen setting is later changed to 70%, the FRC O₂ amountwill automatically move to 80%. Lowering the ventilator oxygen setting,however, will not result in lowering the FRC O₂ amount, thereby avoidingthe possibility of low oxygen breathing gases for the patient. Thealteration of the oxygen content of the breathing gases is carried outin step 208 of FIG. 7. For exemplary purposes, below, an alteration inthe form of an increase to 75% O₂ is shown in FIG. 5.

Next, the clinician must select the frequency, or interval, at which thefunctional residual capacity measurements will be carried out. This isperformed at step 210. A single functional residual capacitydetermination by the present method may be selected by the appropriatefield 212 in menu 68. Alternatively, a series of FRC determinations orcycles may be selected, with a series interval, set in field 214,between each determination. The interval may be between one and twelvehours in increments of one hour. The time when the next functionalresidual capacity determination begins is shown in field 215.

Alternatively, functional residual capacity measurements can be set tooccur automatically in conjunction with certain procedures controlled byventilator 10, such as immediately prior and/or after a period ofnebulized drug therapy, recruitment maneuvers, a suction procedure, or achange in ventilator setting. Functional residual capacity measurementmay be initiated, terminated, delayed, interrupted, or prevented inaccordance with the occurrence of events, such as those noted above,that may affect the accuracy of the functional residual capacitymeasurement. For example, a functional residual capacity measurement maybe terminated for a high oxygen procedure for patient 12 and thenresumed or started after a “lock out” period.

The initial or base line amount of nitrogen in the expired breathinggases is determined at step 216. As noted above this may be determinedby subtracting the amounts of oxygen and carbon dioxide, as determinedby gas analyzer 70, from the total amount of the breathing gases, asdetermined using flow sensor 62.

While the present invention is described using nitrogen as the inertgas, it will be appreciated that other inert gas may also be used. Forexample, the breathing gases for patient 12 may include the inert gashelium and amounts of helium expired by the patient could be used in afunctional residual capacity measure in the manner described herein.

To commence the determination of functional residual capacity, breathinggases having the increased amount of oxygen shown in data field 105 areprovided to patient 12 in step 218. The increased percentage of oxygenin the breathing gases will wash a portion of the nitrogen or otherinert gas out of lungs 38 of patient 12 with each breath of the patient.The amount of breathing gases inspired and expired by patient 12 witheach breath, i.e. the tidal volume, is a lung volume that is in additionto the residual volume of the lungs found after expiration. The tidalvolume is also smaller than the residual volume. For a healthy adult atypical tidal volume is 400-700 ml whereas the residual volume orfunctional residual capacity is about 2000 ml. Therefore, only a portionof the nitrogen in the lungs 38 of patient 12 is replaced by theincreased amount of oxygen with each breath.

The amount of nitrogen washed out of the lungs in each breath isdetermined by subtracting the amount of oxygen and carbon dioxide fromthe amount of breathing gases expired by patient 12 during each breathobtained using flow sensor 68. See step 220. Knowing the amount ofexpired breathing gases, the initial amount of expired nitrogen and theamount expired in each expiration by patient 12, a functional residualcapacity quantity can be determined for each successive breath in steps222 a, 222 b . . . 222 n. Any inert gas wash out/wash in functionalresidual capacity measurement technique may be used, a suitabletechnique for determining functional residual capacity for use in thepresent invention being described in U.S. Pat. No. 6,139,506.

The functional residual capacity quantity as determined after eachsuccessive breath, will tend to increase as nitrogen continues to bewashed out of the lungs of the patient by the increased oxygen in thebreathing gases. This results from the fact that the breathing gasesthat are inspired by patient 12, i.e., the tidal volume, are not fullyequilibrated inside the entire functional residual capacity volumebefore being exhaled by the patient. In particular, functional residualcapacity volume that lies behind intrinsic lung resistance does not mixas quickly with inspired gases compared to functional residual capacityvolume that is pneumatically connected to the trachea through a lowerresistance path. As such, the magnitude of breath-to-breath increases infunctional residual capacity that are noted are an indication of theamount of intrinsic resistance within the lung gas transfer pathways.Thought of another way, additional functional residual capacity volumethat is registered many breaths into the functional residual capacitymeasurement procedure is lung volume that is not participating well inthe gas transfer process.

As the determination of functional residual capacity proceeds, thedetermined values for functional residual capacity for the breaths aredisplayed in graphic portion 110 of screen 102 g 2 as a capacity orvolume curve 224 in steps 226 a, 226 b . . . 226 c at the end of thedetermination for each breath. This confirms to the clinician that thedetermination of functional residual capacity is working properly. Also,as curve 224 forms from left to right, the shape of the curve is anindication to the clinician of the intrinsic resistance and quality ofventilation of lung functional residual capacity, as discussed above. Inthe example shown, the clinician can appreciate that patient 12 has ahomogeneously ventilated lung volume, as indicated by the qualitativeflatness of the functional residual capacity curve, with a lung capacityof about 2500 ml.

The scaling of graph 110 of FIG. 5 may be automatically altered toprovide a scale appropriate to the functional residual capacity databeing shown.

It will be appreciated that, if desired, the data relating breath numberto the corresponding functional residual capacity value can also bedisplayed in tabular form in portion 112 of display portion 102 g. Thiscould comprise a column containing the breath numbers and a columncontaining the corresponding functional residual capacity values.

Mechanical ventilator 10 continues to supply breathing gases havingincreased oxygen concentration for x number of breaths, for example, 20breaths. A final value for functional residual capacity is determined atthe end of the x breaths at step 228 and volume or capacity curve 224extends to this breath to show the final determination of functionalresidual capacity at the end of 20 breaths. The functional residualcapacity measurement may conclude earlier if sufficient stability ofbreath-to-breath functional residual capacity is found in curve 224.

Thereafter, at step 230 the concentration of oxygen in the breathinggases is altered to the original level of, for example 50%, set at step208 and ventilator 10 is operated at step 232 to repeat steps 216-228 tomake a second determination of functional residual capacity with thisalteration of the oxygen concentration in the breathing gases. It willbe appreciated that this determination uses a wash-in of nitrogen,rather than a wash-out. This second determination is graphed anddisplayed in graphic portion 110 as graph 234, in the same manner asgraph 224, described above. The values for the two final functionalresidual capacity determinations are shown in data field 237 of tabularportion 112 of screen 102 g 2 in step 236. In the example shown, thesevalues are 2500 and 2550 ml.

For future use, the final determination of functional residual capacitymade in step 232 is compared to that determined in step 228. This iscarried out at step 238. It is then determined, in step 240, whether thedifference between the two determinations of functional residualcapacity is less or greater than some amount, such as 25%. If thedifference is less than 25%, the two values are averaged and will besubsequently displayed in text form in data field 245 in step 244 whendetermination becomes part of the chronological record following a laterfunctional residual capacity determination.

If the difference between the two values for the functional residualcapacity is greater than some amount, such as than 25%, both the finalvalue determined at step 228 and the final value determined in step 232will be displayed by step 246 in data field 245 of FIG. 5 and in thegraph 110. This display of the functional residual capacitydetermination informs the clinician that the accuracy of the functionalresidual capacity determination is questionable.

The final value(s) for the functional residual capacity are preferablydisplayed in tabular portion 112 of screen 102 g 2 along with additionalassociated data such as the time and date at which functional residualcapacity was determined, or the values of PEEPe and PEEPi existing whenthe functional residual capacity determination was made. PEEPe is theend expiratory pressure established by ventilator 10. PEEPi, also knownas auto PEEP, is the intrinsic end expiratory pressure and is ameasurement in pressure of the volume of gas trapped in the lungs at theend of expiration to the PEEPe level.

While the determination of functional residual capacity has beendescribed as being carried out for a given number of breaths, such as20, it can be terminated sooner if it is apparent that the functionalresidual capacity measurement has become stable on a breath-to-breathbasis. This can be conveniently determined by measuring the O₂ contentof the expired breathing gases at the end of the patient's expirations,that is, the end tidal oxygen level. When the amount of oxygen in theexpired breathing gases attains and remains at the altered level, it isan indication that the wash out/wash in the inert gas is complete andthat the functional residual capacity determination can be terminated.

Thereafter, if a series of functional residual capacity determinationshas been selected at step 210, steps 218 through 246 are repeated afterthe time interval indicated in data field 214 with the start of thefunctional residual capacity determination occurring at the timedisplayed in data field 248. The predetermined time interval may beoverridden or the functional residual capacity determination terminatedby appropriate commands from the clinician entered into menu 68.

The volume curves, such as 224, 234, and functional residual capacitydata, such as that in field 237, generated in the course of successivefunctional residual capacity determinations are saved by ventilatordisplay unit 76 and, as such, can be compared to data from previous orsubsequent functional residual capacity determinations. This comparisonrequires that a previous determination of functional residual capacitybe selected as a reference curve using the time at which it was obtainedas identified in data field 250. When a reference curve is selected, anindication is made in data field 250 and that functional residualcapacity curve is displayed as the reference curve 252. Curve 252 showsa lung that is not well ventilated. Further indication of the referencecurve and reference curve values may be made by a color indication forthis data, different from that of the other functional residual capacitydata in graph 110 and table 112. The result is a visual indicator thatcan easily be referred to by the clinician to quickly assess improvementor deterioration in the functional residual capacity condition ofpatient 12 over time. In the example shown in FIG. 5, there has been anincrease in the functional residual capacity of patient 12 for eacheight hour interval.

Also, it is common practice to alter, usually increase, the PEEP toimprove ventilation of lungs 38 of patient 12 by opening areas of thelung that are not being properly ventilated. Tabulating the actualmeasured values for PEEPe and PEEPi, along with the correspondingfunctional residual capacity determination, as shown in FIG. 5, allowsthe clinician to see the effect, if any of applied PEEPe therapy on thevolume of the functional residual capacity of the patient's lungs, aswell as on the intrinsic PEEP. As also shown in FIG. 5, a history of acertain number of functional residual capacity determinations and PEEPpressures are shown in display region 70 to present trends and thehistory of these quantities. In the example shown there, an increase inPEEPe has resulted in an increase in functional residual capacity ofpatient 12.

FRC Events Log

Certain clinical or other events can affect the value for functionalresidual capacity determined from the method steps shown in FIG. 7. Suchevents may include performing a suction routine on patient 12 to removeaccumulated secretions, administering a nebulized medication, changingthe ventilation mode, or changing one or more ventilation parameters,such as tidal volume (TV), breath rate, PEEP, or other parameter.

By selecting the FRC Log field 252 in menu 68 of screen 102 g 2 shown inFIG. 5, screen 102 g 4 of FIG. 8 will be shown to provide a log of theevents that may effect functional residual capacity in data field 254along with the time(s) and date(s) the event took place. The log alsoincludes the time, date and value of any periodic functional residualcapacity determinations made in the manner described above. Theclinician may scroll through the events of the log using control knob106 to review the functional residual capacity event history in relationto the measured values of functional residual capacity to determine ifspecific actions had a positive or negative effect on the determinedfunctional residual capacity for the patient.

PEEP Determination and Display

An aspect of the present invention allows the clinician to ascertain therelationship between the functional residual capacity of patient 12, andPEEP applied to the patient, thereby to assist the clinician inestablishing a PEEP level deemed optimal for patient 12. An optimal PEEPlevel, in the present context, is one beyond which diminishingfunctional residual capacity increases in association with PEEPincreases is noted. The PEEP INview screen 102 g 5 of FIG. 9 may be usedfor this purpose. For screen 102 g 5, a series of periodic functionalresidual capacity determinations is made, preferably in the manner shownin FIG. 7, with each determination being at a different incrementedlevel of PEEP. For this purpose, in menu 108, the clinician sets analtered concentration of oxygen to be used in the functional residualcapacity determination in data field 400. The clinician also enters aninitial PEEP value at data field 402 and an end PEEP value at data field404. The initial PEEP value may be low and the end value high, as shownin FIG. 9, or the initial value high and the end value low. Theclinician also sets the number of functional residual capacitymeasurements to be made between the initial and end PEEP values in datafield 406. In the example shown in FIG. 9, five such measurements are tobe made. In an alternative embodiment, the incremental PEEP associatedwith each measurement, for example an incremental change of 3 cmH₂O foreach measurement, may be set. While use of the method of determiningfunctional residual capacity of FIG. 7 is described below, it will beappreciated, that for the purpose of determining a suitable PEEP, anymethod of determining functional residual capacity may be employed.

The series of measurements of functional residual capacity starting atthe initial value of PEEP and incrementally moving to the end value ofPEEP is then performed as in a manner of steps 216-228 or steps 216-246shown in FIG. 7. These functional residual capacity determinations aregraphically displayed in graph 110 of FIG. 9 as points/curve 408. Graph110 of screen 102 g 5 has functional residual capacity on the ordinateand PEEP on the abscissa. The corresponding numeric functional residualcapacity data is displayed in table 112 that contains the functionalresidual capacity values and the PEEP values at which that functionalresidual capacity value was obtained.

Curve 408 and table 112 provide guidance to the clinician in selecting aPEEP level for ventilating patient 12. For example, from the graph andtable of FIG. 9 it can be seen that increasing the PEEP from 2 to 6cmH₂O increases functional residual capacity by 500 ml whereasincreasing PEEP beyond 6 cmH₂O provides a relatively small increase infunctional residual capacity. This suggests to the clinician that 6cmH₂O would be an appropriate PEEP for the patient.

Curve 408 can be saved in a memory in ventilator 10 or display unit 76.If ventilator settings are not changed or are not changed in anysignificant way, curves 408 obtained at different times in the course ofthe patient's treatment can be usefully presented in graphic portion 110of screen 102 g to enable the clinician to note changes in the PEEPcurve over time by comparing the data of two or more curves 408 overtime.

Also, while the foregoing has described obtaining and presenting a graphand table of functional residual capacity and PEEP, other aspects of theventilation of patient 12 by ventilator 10 may affect the functionalresidual capacity. For example, the respiration rate, or the relatedquantities of expiration time and inspiratory:expiratory (I:E) ratio,can affect functional residual capacity primarily through the mechanismof intrinsic PEEP. Determining and displaying the relationship of one ormore of these quantities to functional residual capacity may be usefulto a clinician. To this end, the functional residual capacity of thelungs 38 of patient 12 can be determined at differing respiration ratesand the data displayed in graphic or tabular form to show therelationship between functional residual capacity and respiration rate.In graphic portion 110, the abscissa would show the respiration ratewhile the abscissa continues to show functional residual capacity. Atabular presentation comprises a column of respiration rates and acolumn of corresponding functional residual capacity determinations.

FIG. 9B shows an example of a further manner of obtaining and displayingfunctional residual capacity and PEEP data. In FIG. 9B, the abscissapresents PEEP and the ordinate presents the change in functionalresidual capacity volume for a given change in PEEP, or ΔFRC/ΔPEEP.

The relationship between changes in lung volume and changes in lungpressure is termed the “compliance” of the lung. As a general expressionof lung characteristics, it describes the elasticity or “stiffness” ofthe lungs. Lungs of high compliance are elastic and a large change involume occurs for a small change in pressure. The reverse is true for astiff lung. Some lung conditions decrease lung compliance. Others, suchas emphysema, increase lung compliance.

In the present context, the presentation of changes in functionalresidual capacity volume, ΔFRC, to changes in PEEP, ΔPEEP, in FIG. 9B ascompliance properties of the lung serves to emphasize clinicallyimportant data presented in graphic and textual form in FIG. 9 and toaid the clinician in selecting an appropriate PEEP for the patient. Forthis purpose, the ordinate of graph 110 in FIG. 9B is labeled ascompliance. The data in FIG. 9 presented in the manner of FIG. 9B showsthat the incremental 2 cmH₂O pressure increase in PEEP from 2 to 4 cmH₂OPEEP produced a functional residual capacity volume increase of 200 ml,giving a compliance of 100 ml/cmH₂O, as graphically shown in FIG. 9B atthe left hand point in the graph. In the same manner, the incremental 2cmH₂O PEEP increase from 4 to 6 cmH₂O PEEP produced a functionalresidual capacity volume increase of 300 ml, giving a compliance of 150ml/cmH₂O at the second point, proceeding to the right in the graph ofFIG. 9B. Corresponding determinations are made for the incremental PEEPincreases from 6 to 8 and from 8 to 10 cmH₂O and shown by the remainingpoints in FIG. 9B. The data may also be presented in tabular form intable 112 of FIG. 9B.

The peak in the graph of FIG. 9B suggests to the clinician that a PEEPof 6 cmH₂O would be advantageous for the patient.

Recruitment/De-Recruitment of Lung Volume

While screen 102 g 5 of FIG. 9 provides valuable insight and informationto the clinician, it may also be helpful for the clinician to have abetter idea of how much of an increase in functional residual capacityis due to distension of the lung by increased PEEP and how much is dueto making previously closed alveolar sacs available, i.e., opening ofthe lung by “recruitment” of lung volume.

One way such information may be obtained using the PEEP INview screen102 g 5 of FIG. 9 is as follows. Functional residual capacity isdetermined for a series of PEEPs, in the manner described above toproduce a curve, such as curve 408. Thereafter a recruitment maneuver iscarried out on patient 12 to open the alveolar sacs of the lungs of thepatient. This would ordinarily be the provision of a high level of PEEPthat, while it may only be tolerated by a patient for a short period oftime, serves to open the alveolar sacs of the patient lungs. This isordinarily carried out by the clinician by operating ventilator 10independently of screen 102 g 5. For this maneuver, it is preferable touse a recruitment PEEP greater than the highest PEEP set in menu 108 ofscreen 102 g 5 to ensure the alveolar sacs open.

After the recruitment maneuver has been completed, the functionalresidual capacity is again determined for the same series of PEEPs usedprior to performing the recruitment maneuver to produce another curve408 a. The two curves can be displayed in graphic portion 110 of screen102 g 5 in the manner shown in FIG. 9A. If the recruitment maneuverresulted in the recruitment of lung volume, i.e. in the opening ofpreviously closed alveolar sacs, curve 408 a will show a higherfunctional residual capacity than curve 408, as shown in FIG. 9A. Curves408 and 408 a will tend to come together at the PEEP level at whichde-recruitment of lung volume begins to occur. This suggests to theclinician that a PEEP level greater than one at which de-recruitmentbegins to occur would be appropriate for the patient.

Another way such information can be obtained using the spirometryaspects of the present invention, as shown in the SpiroD screen 102 g 3of FIG. 10 and, particularly, the lung INview screen 102 g 6 of FIG. 12.

In general, spirometry is used to determine the mechanics of a patient'slungs by examining relationships between breathing gas flows, volumes,and pressures during a breath of a patient. A commonly used relationshipis that between inspired/expired breathing gas flows and volumes that,when graphed, produces a loop spirogram. The size and shape of the loopis used to diagnose the condition of the lung.

A relationship also exists between inspired/expired gas volumes andpressure in the lungs. In the past, a problem with the use of thisrelationship has been that pressure has been measured at a point removedfrom the lungs so that the measured pressure may not be an accuratereflection of actual pressure in the lungs thus lessening the diagnosticvalue of the pressure-volume loop. Through the use of catheter 94extending from endotracheal tube 90 shown in FIG. 2, a far more accurateindication of lung pressure is obtained. For a healthy lung, a graph ofthe relationship between volume and pressure is roughly an elongated,narrow loop of positive uniform slope. That is, constant increments ofinspired volume increase lung pressure by constant increments. The loopis formed because there remains some amount of lung resistance below thepressure sensing point at the end of catheter 94. In a diseased lung,the loop may be wider and may also reflect a non-linear lung volumepressure relationship. For such a lung, the volume-pressure relationshipover the course of an inspiration/expiration may be in a form such asthat shown in FIG. 10 by 420, and a curve illustrating thevolume-pressure relationship resulting from a mathematical computationusing loop data is plotted, as shown in FIG. 10 by reference numeral422. The curve 422 shown in FIG. 10 in often termed a “dynostatic curve”and is used for diagnostic purposes. A typical dynostatic curve is shownin FIG. 10 to contain a middle portion of somewhat linear positive slopeand a pair of inflection points separating end portions of differingslopes. The dynostatic curve and its generation is described inPractical Assessment of Respiratory Mechanics by Ola Stenqvist, BritishJournal of Anesthesia 91(1), pp. 92-105 (2003) and “The DynostaticAlgorithm in Adult and Paediatric Respiratory Monitoring” by SorenSondergaard, Thesis, University Hospital, Gothenburg University, Sweden(2002).

In graph 110 of FIG. 10, the abscissa of the graph is lung pressuremeasured at the end of catheter 94 connected to the auxiliary input A ofventilator display unit 76 and is termed “Paux”. The ordinate is scaledin volume of breathing gases inspired/expired by patient 12. It will beappreciated that this volume comprises the tidal volume for the patient.The tidal volume moves into and out of the lungs in a manner that can bedescribed as being “above” the functional residual capacity. That is,for normal breathing, a patient starts a breath with the volume of thelungs at the functional residual capacity which may, for example be 2000ml. During inhalation, the volume of the lungs increases by the tidalvolume of, for example 500-700 ml, and during exhalation, the volume ofthe lungs decreases by approximately that amount. The same situationoccurs when a patient is being provided with breathing gases from amechanical ventilator, such as ventilator 10. It must thus beappreciated that the ordinate of the graph 110 in FIG. 10 is scaled inthe relative volume of inspiration/expiration for which the origin ofthe graph is zero, not in absolute volume that would also take intoconsideration functional residual capacity and for which the origin of agraph would be the amount of the functional residual capacity. Thescaling of graph 110 of FIG. 10 may be automatically altered to providea scale appropriate to the spirometry data being shown.

With PEEP applied to patient 12 by ventilator 10, there will be amovement of the graph away from the origin of the axes along theabscissa. The graph will move right by the amount of the PEEP, i.e. thelung pressure at the end of expiration by patient 12.

The menu portion 108 of SpiroD screen 102 g 3 shown in FIG. 10 allowsthe user to open up a set up menu, shown in FIG. 11 that allows theclinician to turn a purge flow through catheter 94 on or off or to zerothe Paux sensor connected to catheter 94 when the purge flow is on andendotracheal tube 90 has been inserted in patient 12. The SpiroD set-upmenu also allows the clinician to set the scaling for the graphicalportions of the display. A “Paux Alarm” screen, reached from the SpiroDsetup screen of FIG. 11, allows the clinician to set appropriate alarmsfor patient lung pressure, as sensed by catheter 94.

Various other selections on menu 108 of screen 102 g 3 of FIG. 10 allowthe clinician to save the current data and to view this information as afirst or second reference for use and display with subsequently obtaineddata. Up to a given number of loops, for example, six loops and curves,may be saved for analytical purposes. The “erase reference” optionallows the user to determine which information to save and which todelete.

The “SpiroD loops” and “SpiroD curves” menu items may be turned on oroff. Selecting “on” for both the curve and loop will display both theloop and the curve at once in the manner shown in FIG. 10. For easiercomparison among loops and curves obtained at various times, either theloop or curve showing may be turned “off.” The “cursor” option allowsthe clinician to scroll along the horizontal axis and display the actualpressure and volume measurements associated with the loops or curvesthat are displayed.

For the graphical showing of graph 110 of the screen 102 g 3 in FIG. 10,volumes and pressures are obtained from sensor 57 and catheter 94 andthe spirometry data, computed and displayed for every third breath ifthe respiratory rate is less than some desired number, for example, 15breaths per minute. If the respiratory rate is greater than that number,every fifth breath used. The loop 420 for a completeinspiratory/expiratory breathing cycle is displayed in the graph ofscreen 102 g 3 of FIG. 10. The dynostatic curve 422 is then calculatedfor display in graph 110.

Various compliance values for the patient's lungs are shown in the table112 of screen 102 g 3 of FIG. 10. Compliance can be seen as the amountby which the volume of the lung increases for an incremental increase inlung pressure. The data necessary to determine compliance can beobtained from sensor 57 and gas module 64. Compliance is represented bythe slope of dynostatic curve 422. It is an indication of the stiffnessor elasticity of the lung. In a stiff lung, an incremental increase inpressure results in a smaller increase in volume over a lung that ismore elastic and the slope of curve 422 is more horizontal. In anelastic lung, the reverse is true. To aid the clinician in analyzing thelungs of patient 10, the compliance is computed at the beginning,middle, and end of the respiratory cycle of the patient. As shown in theexample in FIG. 10, the middle portion of dynostatic curve 422 indicatesa portion of greater compliance than the end portions. This is reflectedin the greater slope of the middle portion over those of the endportions. The table of the screen sets out numerical values. Ordinarily,the highly compliant, middle portion of curve 422 shown in FIG. 10 isthat in which the lung is most effectively ventilated.

The table 112 of display 102 g 3 of FIG. 10 also shows the peak pressureachieved in the lungs during the breath, the PEEP pressure, and theairway resistance, Raw. The airway resistance is the pressure dropexperienced by breathing gas flow of the lungs and is expressed in unitsof pressure per unit of flow. Airway resistance can also be determinedwith data from sensor 57 and gas module 64 in a manner described in theStenqvist reference noted above.

The present invention provides a unique way of viewing the relationshipamong functional residual capacity, PEEP, and recruited:de-recruitedlung volume that is deemed helpful in enabling a clinician to determinea suitable value for PEEP. To carry this out, the PEEP INview screen 102g 5 shown in FIG. 9 is reached. Among the menu items present in PEEPINview screen 102 g 5 is “Lung INview on/off” in field 424. When “LungINview” is turned “on” a screen 102 g 6 in the format of FIGS. 12 a and12 b is present in ventilator display unit 76. As shown in FIG. 4, thePEEP INview screen 102 g 6 of FIG. 9 can be reached either via the FRCINview display 102 g 2 described above and shown in FIG. 5 at field 426or the SpiroD display 102 g 3 shown in FIG. 10 at field 428. When theformer route is chosen to reach the PEEP INview screen of FIG. 9, thespirometry data described above will still be obtained and calculated inventilator display unit 76 but the screen of FIG. 10 will not bedisplayed in the display unit.

To proceed with the Lung INview display, in the PEEP INview screen 102 g5 shown in FIG. 9, the menu item “Lung INview on/off” 424 is toggled“on” and the ventilator display unit will show screen 102 g 6 in theformat of FIGS. 12 a and 12 b.

For an embodiment of the invention using a wash in/wash outdetermination of functional residual capacity, in field 430 of menu 108of display 102 g 6 of FIG. 12 a/b, the clinician sets the altered oxygenlevel to be used in the functional residual capacity measurementemployed to produce the Lung INview data. To begin the process ofproviding data for display 102 g 6, the clinician establishes initialand end PEEPs values at fields 432 and 434, as well as the number ofmeasurements to be taken within the range of PEEP so established atfield 436, in the same general manner as described above in connectionwith the PEEP INview display 102 g 5 of FIG. 9. Alternatively, and ifdesired, field 436 can show the incremental/decremental PEEP step, forexample a step of 3 cmH₂O. In the example shown in FIG. 12, the initialPEEP is 25 cmH₂O, the end PEEP is 5 cmH₂O, and five measurements will betaken within that range of PEEP, namely measurements at 25, 20, 15, 10,and 5 cmH₂O of PEEP. It is deemed preferable to initiate the process ofdetermining the optimal PEEP by using the highest value of the selectedrange and thereafter decrementing the applied PEEP to the lower endlevel. However, as noted in connection with the description of screen102 g 5 of FIG. 9, the invention can also be practiced with incrementingPEEP from a low value to a high value, depending on the preference ofthe clinician.

The graph 110 of screen 102 g 6 of FIG. 12 has the abscissa scaled inPEEP and the ordinate scaled in functional residual capacity. The table112 of screen 102 g 6 provides columns for the set PEEP, functionalresidual capacity and airway resistance (Raw). The table also includes acolumn for “difference” that, as hereinafter described, contains anumerical indication of the lung volume that is recruited/de-recruitedin the lungs as the PEEP is incremented/decremented.

As shown in FIG. 13, at step 500, a recruitment maneuver is preferablyrun on patient 12 to open the lungs of the patient. As noted above, suchrecruitment maneuver would typically be the provision of a high level ofPEEP opens the alveolar sacs of the patient lungs. It is preferable touse a recruitment PEEP greater than the highest PEEP set in menu 108 ofscreen 102 g 6 to ensure the alveolar sacs open. In step 502, thefunctional residual capacity and PEEP parameters are established in menu108 of display 102 g 6, shown in FIG. 12, as previously described. Step502 may occur before step 500 reversing the order shown in FIG. 13.Thereafter, in step 504, the patient is ventilated by ventilator 10 withthe PEEP at the initial level found in the menu. In the presentexemplary instance, patient 12 is initially ventilated with a PEEP of 25cmH₂O in step 504.

In step 506, the functional residual capacity of patient 12 isdetermined by the wash in/wash out technique using the altered oxygenconcentration level as described above in connection with FIGS. 5 and 7or using some other appropriate technique for measuring functionalresidual capacity. The determined value is graphically displayed ingraph 110 at the corresponding value of PEEP as point 508. The valuesare also entered in tabular form in table 112 of screen 102 g 6 of FIG.12.

Next, the PEEP pressure is altered to the next decremental level, in thepresent instance from 25 cmH₂O to 20 cmH₂O, and the patient isventilated at the new PEEP level in step 510. In step 512, thefunctional residual capacity is again determined and displayed withrespect to PEEP in the same manner as in step 506 at point 514. Curve516 is formed in graph 110 from points 508 and 514.

A dynostatic curve is also obtained for the ventilation of the patient'slungs at the PEEP of 20 cmH₂O. FIG. 14 shows the spirometry dataobtained at step 518 for the PEEP of 20 cmH₂O. For ease of explanation,the spirometry loop is shown simply as an ellipse 520 and dynostaticcurve 522 shown as a straight line, it being understood that thespirometry loops and curves will actually resemble those shown in FIG.10. However, the ordinate of the graph of FIG. 14 is scaled in absolutevolume, not the relative volume of FIG. 10, so as to show functionalresidual capacity, as noted in steps 506 and 512.

The origin for dynostatic curve 522 will be the PEEP value of 20 cmH₂Oand the associated functional residual capacity value so that the origincorresponds to point 514 of FIG. 12 and FIG. 14. FIG. 14 can be seen asan enlargement of FIG. 12 showing the data used to generate the data ofFIG. 12 and also showing dynostatic loops and curves. FIG. 14 shows asignificant amount of data relating to the condition of the lungs ofpatient 12. The graph 110 of screen 102 g 6 of FIG. 12 shows the salientfeatures of that data, thereby to assist and facilitate the selection ofan optimal PEEP for patient 12 by the clinician.

Steps 510, 512 and 518 are then repeated for the next decremented PEEPof 15 cmH₂O. This produces a new point 530 of functional residualcapacity and PEEP in curve 516 in FIGS. 12 a and 12 b and in FIG. 14. Italso produces a new spirometry loop and dynostatic curve 532. Steps 510,512, and 518 are again repeated for a PEEP of 10 cmH₂O to produce point534 and dynostatic curve 536 shown in FIG. 14.

Steps 524, 526 and 528 are used to determine the amount of recruited orde-recruited volume obtained in the lungs of patient 12. At PEEPs of 15and 10 cmH₂O, some de-recruitment of lung volume is noted and steps 524,526 and 528 of FIG. 13 are explained in conjunction with these PEEPs. Instep 524 and using the dynostatic curve 536 for the reduced PEEP of 10cmH₂O, the volume of the lungs at a pressure corresponding to that ofthe previous PEEP of 15 cmH₂O is determined. Graphically, this may beaccomplished by establishing vertical line 538 in of FIG. 14 at theprevious PEEP of 15 cmH₂O and noting the intersection of line 538 anddynostatic curve 536 at point 540.

The amount of volume on the ordinate scale represented by the linesegment 538 a between points 530 and 540 is also determined. In thepresent instance, this amounts to approximately 180 ml. In step 528,this value is placed in table 112 of screen 102 g 6 in association withthe previous PEEP of 15 cmH₂O. In step 526, point 540 is placed in graph110 of screen 102 g 5 as shown in FIGS. 12 a and 12 b.

FIG. 12 b shows the completed Lung INview process, including the finalmeasurement of functional residual capacity at a PEEP of 5 cmH₂O. Thisis carried out by repeating steps 510, 512, and 518, for a PEEP of 5cmH₂O to produce plot 542 of functional residual capacity and PEEP anddynostatic curve 544. Repeating steps 524, 526, and 528 produces point546 and line segment 548 a representing a volume of about 120 ml. Thedata is displayed in a manner corresponding to that described above ingraph 110 and table 112 of screen 102 g 6. As the determination of the“difference” requires both functional residual capacity measurementtaken at a previous PEEP and a dynostatic curve from subsequent PEEP andis referenced to the previous PEEP, no difference value will appear inthe graph and table of screen 102 g 6 of FIG. 12 b for the 5 cmH₂O levelof PEEP.

Reverting now to the situation with respect to the PEEPs of 25 and 20cmH₂O, as noted above, at these higher PEEPs, there is littlede-recruitment of lung volume as the alveolar sacs are continuously openduring the respiratory cycle. This is expressed in FIG. 14 by the factthat dynostatic curve 522 for a PEEP of 20 cmH₂O passes through point508 formed using the functional residual capacity for a PEEP of 25cmH₂O. Thus, no difference of the type represented by lines 538 a and548 a will be seen when the PEEP is reduced from 25 cmH₂O to 20 cmH₂O.This fact is shown as 0 difference in table 112 for a PEEP of 25 cmH₂O,since, as noted at step 528, the difference is tabulated to the previousPEEP.

An analogous situation exists for dynostatic curve 532 generated for thePEEP of 15 cmH₂O. That is, dynostatic curve 532 passes through point 514formed using the functional residual capacity for 20 cmH₂O. Again, thereis a zero difference as tabulated in table 112 for 20 cmH₂O. In graph110 of FIGS. 12 a and 12 b, where the difference approximates zero, thedifferences determined in step 528 and the plot of the functionalresidual capacity at the previous PEEP are roughly the same andoverlapping.

However, as the PEEP is further decremented, de-recruitment of lungvolume begins to occur. For example, point 530 shows that for a PEEP of15 cmH₂O, when the lung pressure is at that level, the lung volume isabout 1800 ml. But when the PEEP is lowered to 10 cmH₂O, for a pressurein the lungs of 15 cmH₂O, the lung volume is only about 1620 ml, asevidenced by the plot of point 540. There has thus been a lung volumede-recruitment of approximately 180 ml when the PEEP was lowered from 15cmH₂O to 10 cmH₂O, as evidenced by the length of line segment 538 a.

An analogous situation exists when the PEEP is lowered from 10 cmH₂O to5 cmH₂O as shown by line segment 548 a. The de-recruitment of lungvolume in that case is about 120 ml.

It will be appreciated, that a clinician may readily discern an optimalPEEP for patient 12 from the graphic and tabular data provided in screen102 g 6 of FIG. 12 b. Right after the recruitment maneuver of step 500,and for the higher PEEPs of 25 and 20 cmH₂O, the alveolar sacs willremain generally open during breathing due to the higher pressures.While this is advantageous from the standpoint of lung volume, the highpressure may be injurious to the patient. There is little reduction, orde-recruitment of lung volume as expiration proceeds to the endexpiratory pressure, as shown in FIG. 12.

As PEEP is further reduced to 15 cmH₂O and then to 10 cmH₂O, adifference in volume will occur and curve 548 will separate below curve516 in graph 110. The clinician will be able to note that at a PEEP of10 cmH₂O, a portion of the lung volume that had been open at a PEEP of15 cmH₂O will remain closed as pressure is increased from the PEEP of 10cmH₂O to 15 cmH₂O during the course of inspiration while moving up thedynostatic curve. This difference, 180 ml in the example shown in FIGS.12 a and 12 b and line segment 538 a of FIG. 14, represents the“de-recruitment” of lung volume as the PEEP was reduced from 15 cmH₂O to10 cmH₂O. Conversely, a volume would be “recruited” if the PEEP wasincreased from 10 cmH₂O to 15 cmH₂O.

Further lowering the PEEP to 5 cmH₂O results in an additionalde-recruited loss of lung volume of 120 ml as shown by line segment 548a. As can be seen from the graph, the lung begins to lose volume or“derecruits” at PEEP settings below 15 cmH₂O and this suggests that 15cmH₂O is a PEEP that is best suited or optimal for patient 12. Inselecting an optimal PEEP, the clinician may set the PEEP at 15 cmH₂O soas to obtain some recruitment of lung volume over a PEEP of 10 cmH₂O.This may be preceded by a recruitment maneuver, such as at step 500, ifdesired. Or, the clinician may leave the PEEP at 10 cmH₂O since somerecruitment is obtained at that level of PEEP.

As described above in connection with the determination of functionalresidual capacity, the determination of a suitable PEEP can be set toautomatically occur in conjunction with certain procedures carried outby ventilator 10 or treatment procedures carried out on patient 12.

While the foregoing describes an example in which PEEP is decreased asthe amount of recruitment or de-recruitment is determined, it will beappreciated that the technique may also be carried out usingincremented, increasing values of PEEP.

Various alternatives and embodiments are contemplated as being withinthe scope of the following claims particularly pointing out anddistinctly claiming the subject matter regarded as the invention.

1. A ventilator for ventilating a patient and for determining arelationship between a ventilator parameter and the functional residualcapacity of the patient, said ventilator comprising: (a) means forcausing alteration in a parameter of the ventilation supplied to thepatient; (b) means for determining the functional residual capacity ofthe patient at a plurality of parameter values; (c) means for displayingdata relating functional residual capacity to corresponding parametervalues.
 2. The ventilator according to claim 1 wherein means (a) isfurther defined as means for causing an alteration in the PEEP suppliedto the patient, means (b) is further defined as means for determiningthe functional residual capacity of the patient at a plurality of PEEPs,and means (c) is further defined as means for displaying data relatingfunctional residual capacity to corresponding PEEP levels.
 3. Theventilator according to claim 2 wherein means (a) is further defined asincrementing or decrementing the PEEP.
 4. A ventilator according toclaim 1 wherein said means (b) is further defined as determining thefunctional residual capacity of the patient by an inert gas wash-in orwash-out technique.
 5. The ventilator according to claim 2 wherein saidmeans (c) is further defined as displaying data relating PEEP to thecorresponding functional residual capacity in graphic form.
 6. Theventilator according to claim 2 wherein means (c) is further defined asmeans for displaying data relating PEEP to the corresponding functionalresidual capacity in tabular form.
 7. The ventilator according to claim6 wherein the tabular display includes an indication of PEEPe and PEEPiwith the corresponding functional residual capacity.
 8. The ventilatoraccording to claim 2 including means for setting one or more of initialPEEP, end PEEP, number of measurements, or increment/decrement of PEEPalteration.
 9. The ventilator according to claim 1 further defined asincluding means for storing data relating functional residual capacityto corresponding parameter values.
 10. The ventilator according to claim9 wherein said means (c) is further defined as means for displaying dataobtained at different times.
 11. The ventilator according to claim 1wherein means (a) is further defined as means for causing an alterationin one of a respiration rate, expiration time, or inspiratory:expiratoryratio parameter; means (b) is further defined as means for determiningthe functional residual capacity of the patient at a plurality ofdifferent values for the altered parameter, and means (c) is furtherdefined as means for displaying data relating functional residualcapacity to corresponding parameter values.
 12. The ventilator accordingto claim 2 wherein means (a) is further defined as carrying out arecruitment maneuver and means (c) is further defined as displaying dataobtained prior and subsequent to the recruitment maneuver.
 13. A methoddetermining a relationship between a ventilation parameter and thefunctional residual capacity of a ventilated patient, said methodcomprising the steps of: (a) causing alteration in a parameter of theventilation supplied to the patient; (b) determining the functionalresidual capacity of the patient at a plurality of parameter values; and(c) displaying data relating to functional residual capacity tocorresponding parameter values.
 14. The method according to claim 13wherein step (a) is further defined as causing an alteration in the PEEPsupplied to the patient, step (b) is further defined as determining thefunctional residual capacity of the patient at a plurality of PEEPs, andstep (c) is further defined as displaying data relating functionalresidual capacity to corresponding PEEP levels.
 15. The method accordingto claim 14 wherein step (a) is further defined as incrementing ordecrementing the PEEP.
 16. The method according to claim 14 wherein step(b) is further defined as determining the functional residual capacityof the patient by an inert gas wash-in or wash-out technique.
 17. Themethod according to claim 14 wherein step (c) is further defined asdisplaying data relating PEEP to the corresponding functional residualcapacity in graphic form.
 18. The method according to claim 14 whereinstep (c) is further defined as displaying data relating PEEP to thecorresponding functional residual capacity in tabular form.
 19. Themethod according to claim 18 wherein step (c) is further defined asdisplaying an indication of PEEPe and PEEPi with the correspondingfunctional residual capacity.
 20. The method according to claim 14including a step for setting one or more of initial PEEP, end PEEP,number of measurements, or increment/decrement of PEEP alteration. 21.The method according to claim 13 further defined as storing datarelating to functional residual capacity to corresponding parametervalues.
 22. The method according to claim 21 further defined asdisplaying data obtained at different times.
 23. The method according toclaim 13 wherein step (a) is further defined as causing an alteration inone of a respiration rate, expiration time, or inspiratory:expiratoryratio parameter; step (b) is further defined as determining thefunctional residual capacity of the patient at a plurality of differentvalues for the altered parameter, and step (c) is further defined asdisplaying data relating functional residual capacity to correspondingparameter values.
 24. The method according to claim 14 further includingstep of carrying out a recruitment maneuver and step (c) is furtherdefined as displaying data obtained prior and subsequent to therecruitment maneuver.
 25. A ventilator for indicatingrecruitment/de-recruitment of lung volume in association with varyinglevels of PEEP for a ventilated patient, said apparatus comprising: (a)means for ventilating the patient from a ventilator, said meansventilating the patient with a first level of PEEP, altering the PEEP toa second level, different than said first level, and ventilating thepatient with the second level of PEEP; (b) means for measuring thefunctional residual capacity of the patient at one of the first andsecond levels of PEEP; (c) means for determining a spirometry dynostaticcurve for the breathing action of the patient at the other of said firstand second levels of PEEP; (d) means for determining a lung volumedifference for the functional residual capacity at the one level of PEEP(530) and a point (540) on the dynostatic curve for the other level ofPEEP corresponding to the one level of PEEP; and (e) means fordisplaying the lung volume difference as recruited/de-recruited volumein the lungs when changing between the first and second levels of PEEP.26. The ventilator according to claim 25 where means (a) alters the PEEPfrom a higher level to a lower level.
 27. The ventilator according toclaim 25 wherein said means (c) is further defined as determining aspirometry dynostatic curve on an absolute scale using functionalresidual capacity.
 28. The ventilator of claim 25 wherein means (e) isfurther defined as displaying the lung volume differences at variousPEEPs in conjunction with functional residual capacities of the lungs.29. The ventilator of claim 28 wherein means (e) is further defined asgraphically displaying the lung volume differences and functionalresidual capacities.
 30. The ventilator of claim 28 wherein means (e) isfurther defined as displaying the recruited/de-recruited volumes andfunctional residual capacities in tabular form.
 31. The ventilator ofclaim 25 further including means for performing a recruitment maneuveron the patient.
 32. A method for indicating recruitment/de-recruitmentof lung volume in association with varying levels of PEEP for aventilated patient, said method comprising the steps of: (a) ventilatingthe patient with a first level of PEEP, altering the PEEP to a secondlevel, different than said first level, and ventilating the patient withthe second level of PEEP; (b) measuring the functional residual capacityof the patient at one of the first and second levels of PEEP; (c)determining a spirometry dynostatic curve for the breathing action ofthe patient at the other of said first and second levels of PEEP; (d)determining a lung volume difference for the functional residualcapacity at the one level of PEEP and a point on the dynostatic curvefor the other level of PEEP corresponding to the one level of PEEP; and(e) displaying the lung volume different as recruited/de-recruitedvolume in the lungs when changing between the first and second levels ofPEEP.
 33. The method of claim 32 wherein means (a) alters the PEEP froma higher level to a lower level.
 34. The method of claim 32 furtherincluding an initial step of performing a recruitment maneuver on thepatient.
 35. The method of claim 32 wherein step (c) is further definedas determining a spirometry dynostatic curve on an absolute scale usingfunctional residual capacity.
 36. The method of claim 32 further definedas displaying the lung volume differences at various PEEPs inconjunction with functional residual capacities of the lung.
 37. Themethod of claim 36 further defined as graphically displaying the lungvolume differences and functional residual capacities.
 38. The method ofclaim 36 further defined as displaying the recruited/de-recruitedvolumes and functional residual capacities in tabular form.
 39. Themethod of claim 32 further defined as repeating the steps of the methodby ventilating the patient at the other of the first and second levelsof PEEP, altering the PEEP to a third level, and carrying out thecorresponding steps of the method.
 40. A ventilator for ventilating apatient and for determining a relationship between a ventilatorparameter and a functional residual capacity characteristic of thepatient, said ventilator comprising: (a) means for causing alteration ina PEEP supplied to the patient; (b) means for determining the functionalresidual capacity of the patient at a plurality of PEEP values; (c)means for determining the relationship between increments in thealteration in PEEP and resulting differences in functional residualcapacity; and (d) means for displaying data presenting the relationshipdetermined by means (c) to corresponding values of PEEP.
 41. Theventilator according to claim 40 wherein means (a) is further defined asincrementing or decrementing the PEEP.
 42. A ventilator according toclaim 40 wherein said means (b) is further defined as determining thefunctional residual capacity of the patient by an inert gas wash-in orwash-out technique.
 43. The ventilator according to claim 40 whereinsaid means (d) is further defined as displaying data presenting therelationship determined by means (c) to corresponding PEEP values ingraphic form.
 44. The ventilator according to claim 40 wherein means (d)is further defined as means for displaying data presenting therelationship determined by means (c) to corresponding PEEP values intabular form.
 45. The ventilator according to claim 44 wherein thetabular display includes an indication of PEEPe and PEEPi withcorresponding PEEP values.
 46. The ventilator according to claim 40including means for setting one or more of initial PEEP, end PEEP,number of measurements, or increment/decrement of PEEP alteration. 47.The ventilator according to claim 40 further defined as including meansfor storing data relating the relationship determined by means (c) tocorresponding PEEP values.
 48. A method determining a relationshipbetween a ventilation parameter and a functional residual capacitycharacteristic of a ventilated patient, said method comprising the stepsof: (a) causing alteration in a PEEP supplied to the patient; (b)determining the functional residual capacity of the patient at aplurality of PEEP values; (c) determining the relationship betweenincrements in the alteration in PEEP and resulting differences infunctional residual capacity; and (d) displaying data presenting therelationship determined in step (c) to corresponding values of PEEP. 49.The method according to claim 48 wherein step (a) is further defined asincrementing or decrementing the PEEP.
 50. The method according to claim48 wherein step (b) is further defined as determining the functionalresidual capacity of the patient by an inert gas wash-in or wash-outtechnique.
 51. The method according to claim 48 wherein step (d) isfurther defined as displaying data presenting the relationshipdetermined in step (c) to corresponding PEEP values in graphic form. 52.The method according to claim 48 wherein step (c) is further defined asdisplaying data presenting the relationship determined in step (c) tocorresponding PEEP values in tabular form.
 53. The method according toclaim 52 wherein step (d) is further defined as displaying an indicationof PEEPe and PEEPi with the corresponding PEEP values.
 54. The methodaccording to claim 48 including a step for setting one or more ofinitial PEEP, end PEEP, number of measurements, or increment/decrementof PEEP alteration.
 55. The method according to claim 48 further definedas storing data relating the relationship determined in step (c) tocorresponding PEEP values.